The present invention relates to a method for efficient synchronization with the means of physical layer preambles in a communication system. More in detail the invention refers to a method for efficient synchronization of short-range high data rate communication systems that may be used as a wireless local area network and/or as a wireless asynchronous transfer mode (ATM) system as well as a public access system.
There is an increasing demand for short-range high data rate communication systems for wireless local area network (WLAN) and wireless ATM systems. An important feature of the new systems will be the centralised medium access control (MAC), which is intended to lead to an efficient use of the available frequency spectrum. An access point (AP), also called base station, controls the channel access by assigning capacity for both the down-link and up-link, and the mobile terminals (MT), also called terminal adapter (TA), listen and transmit in the respective xe2x80x9cchannelsxe2x80x9d, e.g. time slots. Such a centralised MAC is typically used in cellular mobile radio systems like GSM.
A characteristic of such a new communication system is that protocol data units (PDUs) transport the data. In particular, there exist only a small number of PDU types, e.g. for control information (control PDU) and for the actual data (data PDU), each type having a fixed size. The new communication system is specified as comprising three parts, namely the physical layer, the data link control (DLC) comprising logical link control (LLC) and MAC, and a convergence layer (CL). The CL is the interface between higher layers and the DLC. For example, there may be a CL for TCP/IP, which segments IP packets into (data) PDUs. The DLC layer adds some header information and provides PDUs of fixed size to the physical layer.
The physical layer of the new communication system will be based on orthogonal frequency division multiplexing (OFDM). An OFDM signal consists of a given number (e.g. 64) of subcarriers, which are a sequence of orthogonal narrowband modulated carrier waves, which fill the available spectral bandwidth of a given channel. The central subcarrier is also called the DC subcarrier. The granularity of data units on the physical layer is therefore an OFDM symbol.
There are WLAN systems performing according to various standards. IEEE802.11 is an example and such systems will have a 5 GHz mode, which will include similar physical layer parameters as the system in which the invention is used. However, the IEEE802.11 system is designed for transmission of IP packets by radio. The protocol principles are similar to Ethernet; hence the MAC is very different from the system of the invention. In an IEEE802.11 system, for instance, IP packets or segments thereof with variable length are transmitted.
A MAC frame consists of several fields containing traffic control information for assigning logical channels as well as data blocks. For synchronisation and channel estimation purposes a preamble may be added at the beginning of each block.
To simplify the data format, the preambles of different channels contain repetitive sequences of the same period length, i.e. repetitions with one quarter of a 64 point FFT (Fast Fourier Transform) length. In operation in an environment with other coexisting communication systems, which might interfere with the new communication system, the coexisting systems could as well use the same repetition distance (L) where distance is the length of the repetitive sequence.
Due to the reuse of the same repetition distance, a mobile terminal might experience severe problems in synchronising with the MAC frame structure of a certain AP in a cell, since it can misinterpret the repetition distance used in the BCCH-TS with one that is used e.g. by another TA in the ULCH-TS. If this is the case, the TA should be given means to recover from such erroneous detection events, i.e. get false alarm detection as fast/soon as possible.
A second problem is spectrum requirements which might limit the power spectral density (PSD) in W/Hz in the given frequency band. If the preamble sequence always uses the same subset of sub-carriers, these will have an increased power spectral density for a typical airframe, e.g. every 4th sub-carrier will have an enhanced PSD.
A third problem is that due to standard regulations, the DC sub-carrier cannot be used, so that there is a gap of twice as many sub-carriers around DC in the power spectrum of the repetitive preamble sequences. Hence less frequency diversity is exploited in this part of the spectrum.
The present invention is concerned with solving the above-described synchronisation, power spectral density and frequency diversity problems.
An object of the invention is to provide a method for improved discrimination of repetitive sequences having the same repetition distance and which is robust to various input power levels and frequency offsets.
Another object of the invention is to provide a method which also limits the power spectral density and the frequency diversity in a given frequency band.
The above mentioned objects are met by the invented method according to which the repetitive sequences are modified primarily by changing the phase and the corresponding auto-correlation magnitude and sign are used for the detection and discrimination of the sequences.
Advantages afforded with this invention are that the above-described problems can be solved. The invention improves the discrimination of preambles and increases the robustness of the communication system. Furthermore the invention makes possible a more efficient utilisation of the available frequency spectrum.